/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/GizmoInterfaces.h
For a gizmo with multiple parts, the part identifier establishes a correspondence between a gizmo part and the elements representing that part within the hit target. The valid part identifiers should be defined in the gizmo. Identifier 0 is reserved for the default ID which should be assigned to elements that do not correspond to any gizmo part, such as non-hittable decorative elements.
class IGizmoClickMultiTarget